Method for examining a sample by using a charged particle beam

ABSTRACT

A method for examining a sample with a scanning charged particle beam imaging apparatus. First, an image area and a scan area are specified on a surface of the sample. Herein, the image area is entirely overlapped within the scan area. Next, the scan area is scanned by using a charged particle beam along a direction neither parallel nor perpendicular to an orientation of the scan area. It is possible that only a portion of the scan area overlapped with the image area is exposed to the charged particle beam. It also is possible that both the shape and the size of the image area are essentially similar with that of the scan area, such that the size of the area projected by the charged particle beam is almost equal to the size of the image area.

FIELD OF THE INVENTION

The present invention generally relates to a method for examining asample with scanning charged particle beam imaging apparatus, and moreparticularly to a method for generating an image of a sample by scanningthe charged particle beam over the sample in a tilted angle over a leastscan area that can render the desired image size with reduced aliasingeffect and charging effects, enhanced image contrast and efficiency.

DESCRIPTION OF THE RELATED ART

The scanning charged particle beam imaging apparatus, such as scanningelectron microscope (SEM), is commonly used to form images which revealthe detailed surface information of a sample. A scanning chargedparticle beam imaging apparatus includes at least a charged particlesource to generate the charged particle beam of a certain energy, one ormore electron lenses to focus the charged particle beam into a fineprobe at the surface of the sample, at least a set of deflector to scanthe charged particle beam probe over an area on the sample to excitecharged particles from the sample surface, for example, secondarycharged particles or backscattered charged particles. At least a chargedparticle detector collects these excited charged particles and convertsthem to electrical signals to form images. The strength of the signal(excited charged particles) usually is a function of surface roughness,materials, and/or charging level at the excitation points. Hence, theobtained images can be further processed or analyzed for acquiring awide variety of detailed information or characteristics of the sample,including surface topography, composition and other properties such aselectrical conductivity.

In the semiconductor industry, the scanning charged particle beamimaging apparatus, for example, SEM, is popularly used in differentstages of integrated circuit device fabrication process for CriticalDimension (CD) measurement, defect inspection, defect review, etc. Thesamples to be examined can be a wafer, a mask, or any template used forproducing patterns on wafer, or on mask, or even template forreproducing template itself FIG. 1A illustrates the prior arts ofimage-based wafer examination. The examination can also be applied tomask or template. The purposes of examination can also be CDmeasurement, defect inspection/review, and so on. The wafer 100comprises lots of dies 101, and all dies 101 have substantiallyidentical layout of micro-structure. Herein, to simplify description andfigure, the internal structures of each die 101, such as effective ICdevice area and scribe line, are omitted. Moreover, the examination ofthe wafer 100 usually is focused on only some specific portions 102 ofeach or specific dies 101, which are called regions of interest (ROI)hereafter. The ROI 102 usually corresponds to areas on the wafer 100where the defects tend to appear, where the layout has more variations,where the yield of a fabrication process trends to be low, and so on.Limited by the maximum image size at a given condition, known as Fieldof View (FOV), the charged particle beam imaging apparatus can only viewa small portion of region at one time. In most cases, the FOV can bemuch smaller than the ROI. Thus, multiple image areas 103 need to betaken to cover the entire ROI. Therefore, as shown in FIG. 1B, toexamine the ROI 102 of die 101, the ROI 102 is firstly divided intosub-regions (image areas), A1, A2, . . . based on the FOV. For imagingeach sub-region (image area) 103, the ideal case is that itscorresponding scan area 104 exactly equals to the size of each imagearea 103, but in practices, scan area 104 is usually larger than theimage area 103 in order to tolerate the unavoidable scanning overhead.

In some prior arts as shown in FIG. 1C, the charged particle beam israster scanned in the direction parallel to the die orientation, asindicated by the arrows, over a scan area 104 (usually in a rectangle orsquare shape) to generate an image of image area 103. Herein, the imagearea 103 also corresponds to the FOV of an image not larger than thescan area 104. This process is repeated until the entire ROI arecovered. However, in the normal case of semiconductor devicefabrication, the image area 103 usually consists of micro structuresarranged substantially in the orientation either parallel orperpendicular to the die orientation. If there is at least onemicro-structure with a sharp horizontal edge, scanning the chargedparticle beam along the horizontal edge may have two differentinfluences to the image: (1) at low magnification operation where thecharged particle probe size is smaller than the image pixel size, thefocused charged particle beam probe may not be kept precisely along theedge of this horizontal edge while scanning, leading to inconsistent andabrupt change of edges signals. This would result in stepped edge orirregular zigzag edge in the obtained image, which is called aliasingeffect. (2) at high magnification operation where the probe size andpixel size getting smaller, scanning the charged particle beam along thesharp edge induce too much charging which further restrain the escape ofsecondary charged particles. Thus, the edge sharpness will be smearedand the edge contrast will be reduced too. Both cases will causetroubles to image examination. For automatically defect detection, edgealiasing will be falsely detected as a defect or cause high false rate,while poor edge sharpness and contrast will lower the detectionsensitivity of defects associated with the edge.

In normal semiconductor device fabrication, the layouts of the basiccomponents (microstructures) inside a die at different process layersare organized in such a way that their edges are either parallel orperpendicular to the die edges. Thus there is a high possibility ofscanning the charged particle beam parallel to the pattern edge. Foravoiding the adverse issues (false defects or poor edge sharpness asdescribed above) associated with the parallel-edge scanning, some priorarts (such as U.S. Pat. No. 6,710,342 and U.S. Pat. No. 6,881,956), asshown in FIG. 1D, raster scan the charged particle beam over a scan area104 along a direction that is at a tilted angle neither parallel norperpendicular to the orientation of the dies on the wafer (which iscalled tilt scan). Herein, the scan line is illustrated as the arrowshown. Therefore, because the tilted scan line is not parallel orperpendicular to die orientation, the charged particle beam essentiallywill not be scanned along the edge of microstructures and then theabovementioned adverse effects can be effectively minimized. However,there will be other practical issues arising from the tilt scan. First,because the image area 103 and the scan area 104 has differentorientations but same shape, the scan area 104 must be obviously largerthan a corresponding image area 103 so that the image area 103 isentirely covered by the scan area 104. Then, the required time to scanwhole scan area 104 is obviously increased when the scan rate is fixed,and then the throughput of the scanning charged particle beam imagingapparatus is lowered. Second, when a specific image area 103 isexamined, such as A8, the corresponding scan area 104 must be overlappedwith at least one neighboring image area(s) 103, such as A3, A7, A9 andA13, as shown in FIG. 1D. This is because each image area 103 is closeto other image area(s) 103. Then, when the scan area 104 is scanned forexamining the specific image area 103 (A8), the charged particle beammust also be projected into a portion of the neighboring image area(s)103 (A3, A7, A9 and A13) and potentially change the surface property ofthese neighboring image area(s) 103 (A3, A7, A9 and A13). For example,the projection of charged particle beam may at least induce electricalcharging and burn mark. Hence, when any of the neighboring image area103 (A3, A7, A9 and A13) is examined later, the acquired images may showsome artifacts which are not the image of the original surface but arecaused by the overlapped scanning.

Moreover, as shown in FIG. 1E, if an image area 103 contains periodicalmicrostructures along a specific direction, such as the orientation ofthe image area 103, and the image is also captured in such a way thatthe orientation of the periodic pattern in the image is parallel orperpendicular to the edge of image, it will be straightforward in imageprocessing to examine the microstructures by shifting image one ormultiple periods in one direction and comparing the neighboringrepeating patterns within the corresponding image. However, if the imageis captured in a way that the periodic patterns are not aligned parallelor perpendicular to the edge of image, as shown in image 105, then theconventional image processing can not be directly applied and anadditional processing is needed to convert the tilted image 105 to anon-tilted image 106 where the orientation of the periodic pattern 106becomes parallel or perpendicular to the edge of image.

Accordingly, the examination of a sample by a scanning charged particlebeam imaging apparatus is not effective enough. It is desired to developnew method for more properly and effectively examining a sample by usinga charged particle beam.

SUMMARY OF THE INVENTION

One approach of this invention is a method for examining a sample withscanning charged particle beam imaging apparatus. Herein, a sample isexamined by scanning a charged particle beam on a scan area (area to bescanned) of the sample in a titled angle neither parallel norperpendicular to the orientation of an image area (area to be examinedby analyzing its image) of the sample, with optimized charged particlebeam control to divert or turn off the charged particle beam to keep itfrom reaching the sample when the charged particle beam is positionedoutside the image area. This will keep the adjacent image areasuntouched by the charged particle beam thus avoiding possible issuessuch like electrical charging and burn mark for the adjacent images.

Another approach of this invention is a method for examining a samplewith scanning charged particle beam imaging apparatus. Herein, a sampleis examined by scanning a charged particle beam on a scan area of thesample in a titled angle neither parallel nor perpendicular to theorientation of image area on the sample, with optimized scanning controlto fit the scan area as precise as possible to just contain the imagearea. This not only keeps the adjacent area substantially untouched bythe scanning charged particle beam for avoiding possible adverse effectsuch like charging and burn mark, but also reduces the time spending onscanning outside image area.

A further approach of this invention is a method for examining a samplewith scanning charged particle beam imaging apparatus. Herein, a sampleis examined by scanning a charged particle beam on a scan area of thesample in a titled angle neither parallel nor perpendicular to theorientation of image area on the sample, with optimized scanning controlto fit the scan area as small as possible to just contain the image areaand optimized charged particle beam control to divert or turn off thecharged particle beam from reaching the sample when the charged particlebeam is positioned outside the image area. Hence, even a portion of thescan line still is outside the image area, owing to the practical factthat the scanning charged particle beam imaging apparatus requires aresponse time to switch the scan line of the charged particle beam, thecharge particle beam still is substantially not projected outside theimage area because the required response time for diverting and/orturning off charged particle beam is significantly less than therequired response time for switching the scan line. Clearly, thisapproach has the advantages of both previous approaches.

Furthermore, if the analysis does require a non-tilted image, theinvention can further have an optional image process for converting thetilted image into a smaller non-tilted image. For example, if the imagewill simply be used for comparison with another image of the similarpattern for detecting difference, it is not necessary to convert thetilted image into a non-tilted one. In wafer inspection, this mode ofdefect detection is called die to die mode. For another example, if theimage contains periodical patterns along a direction which is notparallel or perpendicular to the tilted scan direction, and the furtherprocess need to compare the pattern difference between the neighboringrepeating patterns within one image, the image may need to be convertedto non-tilted image for easy comparison. In wafer inspection, this modeof defect detection is known as Array Mode.

One proposed method for examining a sample with scanning chargedparticle beam imaging apparatus comprising the following steps. First,select a tilted scan angle as well as tiled scan size for each scan areato be scanned by the charged particle beam, and then determines themaximum non-tilted scan or say image size obtainable from the tiltedscan area. Second, specify a sampling region over the surface of asample, and divided it into at least one sub-region(s). Herein, eachsub-region is equal to or less than the maximum non-tilted area sizeobtainable from the scan area, and the number of sub-regions correspondsto the number of scan areas need to be scanned. Herein, each sub-regioncan be viewed as an image area. Next, for the tilted scanning processfor acquiring the image of each sub-region, calculate the area need tobe blanked when charged particle beam is positioned within it anddetermines the blanking parameters (starting time and length) andsequence. Then, scan each sub-region at the given tilted angle neitherparallel nor perpendicular to an orientation of a correspondingsub-region. Herein, for each tilted scanning process, the chargedparticle beam is blanked when the charged particle beam is projectedoutside a corresponding image area. Clearly, the actually scan time maynot be reduced for each titled scanning process, but the actual areawhere the charged particle beam is really projected into is reducedclose to the size of each image area. Finally, the tilted imagesacquired by the tilted scanning processes can be used for furtheranalysis, such as defect detection and defect review. Moreover, ifnecessary, the non-tilted images are reconstructed and formed based onthe tilted images from the tilted scanning processes.

Another proposed method for examining a sample with scanning chargedparticle beam imaging apparatus comprising the following steps. First,select a tilted scan angle as well tiled scan size for each scan area tobe scanned by the charged particle beam, and then determines the maximumnon-tilted size obtainable from the tilted scan area. Second, specify asampling region over the surface of a sample, and divided it into atleast one sub-region(s). Herein, each sub-region is equal to or lessthan the maximum non-tilted area size obtainable from the tiltedscanning process, and the number of sub-regions corresponds to thenumber of scan areas need to be scanned. Herein, each sub-region can beviewed as an image area. Next, calculate the effective area for eachtilted scanning process and determine the optimal scan length of eachline, line by line, at least to cover the desired non-tilted image area.Then, scan each sub-region at the given tilted angle. Herein, the scanlines that the charged particle beam is moved along is restrained tocover essentially only the sub-regions. Hence, the actually scan timewill be reduced for each sub-region, and the actual scanned area also bereduced close to the corresponding non-tilted image size. Finally, thetilted images acquired by the tilted scanning processes can be used forfurther analysis, such as defect detection and defect review. Moreover,if necessary, the non-tilted images are reconstructed and formed basedon the tilted images from the tilted scanning processes. Besides,because the required response time for blanking the charged particlebeam is usually smaller than the required response time for switchingthe routes, also because the trajectory of the projected chargedparticle beam usually can not be abruptly changed without any bufferzone, it is optional to blank the charged particle beam for portions ofthe scan lines outside the sub-region right to be scanned, such that thecharged particle beam is almost only projected into the sub-region to beexamined.

In the present invention, the charged particle beam is raster scanned ina rectangular shape (equal length of each line scan) along a directionneither parallel nor perpendicular to the orientation of the image area,the charged particle beam is blanked out from reach the sample when thescan position outside target image area, thus not expose to chargedparticle beam. Moreover, the raster scan lines can be in a rhomboidshape (variable length of each line) to best fit the image area, thusminimized the scanning time and extra area exposed to charged particle.Therefore, the method disclosed in the present invention may not onlyreduce the possibility of incurrence of the aliasing effect on edges butalso increase the throughput and reduce unnecessarydisadvantageousness(s).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1A illustrates a schematic view of a wafer having some dies.

FIG. 1B schematically illustrates the relations among a die, a samplingregion, an image area and a scan area.

FIG. 1C illustrates a schematic view of a prior art, non-tilt scan.

FIG. 1D illustrates a schematic view of another prior art, tilt-scan.

FIG. 1E illustrates a schematic view of a prior art for simply examininga sample by tilt-scan.

FIG. 2A to FIG. 2F briefly illustrates some proposed ways to solve thedisadvantages of the prior arts.

FIGS. 3A to 3H illustrate a series of steps of a method for examining asample according to an embodiment of the present invention.

FIGS. 4A to 4F illustrate a series of steps of a method for examining asample according to another embodiment of the present invention, andFIGS. 4G to 4I illustrate three scanning schemes respectively instead ofthe steps 4 and 5 illustrated in FIGS. 4D and 4E.

FIGS. 5A to 5G illustrate a series of steps of a method for examining asample according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made in detail to the present embodiments of the inventionand the examples illustrated in the accompanying drawings. Whereverpossible, the same reference numbers are used in the drawings and thedescription to refer to the same or like parts.

The disadvantages of the above prior arts can be briefly attributed tothe following reasons: (1) The charged particles beam is projected onand moved along the edge of the microstructures inside the non-tiltedimage area. (2) The tilted scan area is clearly larger than theeffective image area, thus the required time to scan the whole tiltedscan area is clearly longer than the required time to scan the effectivenon-tilted image area. (3) One titled scan area may overlap with severalneighboring image areas and then these neighboring image areas may beprojected by the charged particle beam when they are not examined(especially before they are examined). Hence, the surface conditions ofthese image areas may be degraded or changed before they are examined.

Clearly, the latter prior art(s), tilted scan as shown in FIG. 1D, canimprove the disadvantages of the former prior art(s), non-tilted scan ashown in FIG. 1C. Then, once the disadvantages of the latter priorart(s) in FIG. 1D are improved, the scanning charged particle beamimaging apparatus can more effectively examine the sample without theseconventional disadvantages. Herein, by carefully analyzing the reasonsof the disadvantages of the latter prior art(s), the key issue can beshortly summarized as below: the whole area to be scanned withprojection of charged particle beam is significantly larger than thewhole area where a corresponding image is required for furtheranalyzing.

By further analyzing the current technology of the scanning chargedparticle beam imaging apparatus, some characteristics can be used tosolve the above key issues. First, the current scanning charged particlebeam imaging apparatus usually can blank and/or turn off the chargedparticle beam, if necessary, such that the scanned sample is notprojected by the charged particle beam. Second, the scan lines areflexible and adjustable, such that each scan line can be independent onother scan lines and then both the length and the size of the scan areacan be adjustable.

Accordingly, the invention provides two ways to solve the disadvantagesof these prior arts. FIG. 2A to FIG. 2F illustrates the essentialconcept of the two proposed ways. Herein, the image areas areillustrated as thin continuous lines and the scan area is illustrated asthin dashed lines. Herein, the scan lines of the charged particle beamis illustrated as bold continuous lines when the charged particle beamis projected on the image area and illustrated as bold dashed lines whenthe charged particle beam is not projected on the image area.

First, if a portion of the sample is not projected by the chargedparticle beam in an on-going scanning process, then no charging or burnmark will appear on the portion before the portion is projected in alater scanning process. Hence, as shown in FIG. 2A and FIG. 2C, aportion of a scan area 104 not overlapped with a corresponding imagearea 103 is not projected by the charged particle beam when thecorresponding image area 103 is examined In such situation, at least oneneighboring image area overlapped with the portions of scan area 104which are not projected by the charged particle beam will not beaffected before the neighboring image area(s) is examined later.Moreover, FIG. 2B and FIG. 2D separately illustrates an application ofFIG. 2A and FIG. 2C; both FIG. 2B and FIG. 2D illustrate a samplingregion consisting of a plurality of image areas 103 located adjacent toeach other. In FIG. 2B, two opposite vertexes of an image area 103 arelocated on two opposite sides of a corresponding scan area 104. Clearly,in FIG. 2C each image area 103 is imaged by using a corresponding scanarea 104 in the way shown in FIG. 2A. After all image areas 103 havebeen scanned, each image area 103 is clearly to be projected by thecharged particle beam only once. In other words, no overlappedprojection is performed to the same image area 103. Hence, theconventional disadvantage of samplings affected by each other iseffectively improved, even prevented. In FIG. 2D, two opposite sides ofan image area 103 are located on two opposite sides of a correspondingscan area 104. Clearly, with the geometric relation between an imagearea 103 and a corresponding scan area 104 applied, this case also canavoid disadvantage induced by repeat projection of charged particle beamon the same image area 103. In short, FIG. 2B and FIG. 2D not onlyillustrate how the disadvantageousness induced by repeat projection ofcharged particle beam can be solved, but also emphasize that thegeometric relation between an image area 103 and a corresponding scanarea 104 is not limited.

Second, as shown in FIG. 2E, if the size of a scan area 104 is almostequal to the size of the image area 103, then the scan time effectivelyspent on generating an image of area 103 is almost equivalent to thescan time to cover the minimum scan area 104. Hence, by properlyadjusting the shape and the size of the scan area 104 to almost fit theshape and the size of the image area 103, the time wasted on scanportion of scan area 104 not overlapped with the corresponding imagearea 103 to be examined is minimized. Then, the throughput iseffectively enhanced. Moreover, FIG. 2F illustrates an application ofFIG. 2E, when FIG. 2F shows illustrate a sampling region consisting of aplurality of image areas 103 located adjacent to each other. In FIG. 2F,each scan area 104 is slightly larger than a corresponding image area103, such that each scan area 104 is slightly overlapped with at leastone adjacent scan area 104. Clearly, because the size of a scan area 104is almost equal to the size of a corresponding image area 103 and eachimage area 103 is totally not overlapped with any neighboring image area103, the size of the overlapped portions among adjacent scan areas 104should be significantly smaller than the size of these image areas 103to be examined Clearly, the required time to scan all scan areas 104 isalmost equal to, only slightly larger than, the required time to scanall image areas 103 to be examined Therefore, the conventionaldisadvantage that the throughput is decreased by scanning more areasthan areas should be examined is effectively improved, even prevented.

One main difference between the invention and the prior arts oftradition approaches can be briefly disclosed as below. In the priorarts, the area to be scanned for acquiring an image is substantiallyidentical to the area where the image is actually formed. Hence, in theprior arts, essentially only the area to be analyzed by image process isscanned. In contrast, for the two proposed approaches, the area to bescanned for acquiring image can be significantly different from the areawhere the image is actually formed as well. Hence, for the proposedapproaches, the area to be analyzed can be significantly different fromor substantially equal to the area to be scanned. Accordingly, FIGS.2A˜2D and FIGS. 2E˜2F separately illustrate the relationship of scanarea and image area of an general tilted scan image, as well as how thesampling region are covered or divided based on the designed image areaand scan area. As noticed from the figures, if the whole sampling regionis to be covered by tilted scan images, the scan areas can be overlappedeach other for neighboring images and even exceed outside the samplingregion, while the image areas can be well stitched and contained withinthe sampling region. In such way, the whole sampling region can beimaged and examined in sequence by multiple tilted images.

Therefore, as these prior arts are focused on the “title scan”, when asampling region to be examined is assigned, the invention may firstlyselect a tilted scan angle as well as a tilted scan size, and thendetermines a maximum non-titled image size obtainable from a titled scanarea with the tilted scan angle and the tilted scan size. Next, dividethe sampling region into some sub-regions, which may be viewed as imageareas. Herein, for different image areas, the corresponding tilted scanareas may be equal or different, although it is more practical to haveequivalent tilted scan areas for different image areas. Herein, themaximum titled scan size is limited by the maximum F.O.V. of thescanning charged particle beam imaging apparatus at a given condition.Herein, for each image area, the size of the image area is selected notto be larger than the maximum non-tilted image size a correspondingtitled scan area can accommodate. After that, for each image area, theoptimal scan lines, usually decided by two main parameters, startingpositions and lengths, are determined so that the formed tilted scanlines at least cover the desired non-tilted image area, even to coverthe whole scan area. Finally, each image area is scanned line by linewith corresponding optimal starting positions and lengths, and thenimaging of the whole sampling region is accomplished after all imageareas are scanned. Clearly, the image acquired by scanning the samplingregion can be used to review and/or detect any defect on the samplingregion. Further, owing to the practical requirement of scanning back, asusual, the size of the scan area is not equal to the size of the imagearea, but is at least slightly larger than the size of the image area.

Moreover, by comparing FIGS. 2A-2D with FIG. 1D, a characteristic of theinvention is clear. In the conventional “tilt scan”, the chargedparticle beam is always projected on the scan area when different tiltedscan lines are formed in sequence. In other words, in the conventional“tilt scan”, scan along a scan line means all points (or pixels) on thescan line are projected by the charged particle beam in sequence. Incontrast, in some examples of the invention as shown in FIGS. 2A-2D, thecharged particle beam is only projected on portion of the scan area whendifferent tilted scan lines are formed in sequence. In other words, inthese examples, scan along a scan line does not express how the chargedparticle beam is projected on the scan line. Hence, it should beexpressed as “scan with projection of charged particle beam” and/or“scan with blanking or turning off the charged particle beam” toprecisely describe how different portions of the scan area are imaged bythe charged particle beam. In the below description of the patentspecification, such expressions are used to describe the invention.

Furthermore, as shown in FIG. 2A to FIG. 2F, after an end point of ascan line is scanned for acquiring image, the starting point of a nextscan line is the next point to be scanned for acquiring image. In otherwords, these scan lines are scanned line by line. Clearly, a scanningback process is required to move from the end point of a former scanline to the starting point of a latter scan line. Herein, the scanningback process needs not to acquire any image signal. In other words, itneeds not to collect and analyze any charged particles, such aselectrons, from the sample. Hence, to compare the scanning process alongthe scan line for acquiring image by collecting and analyzing chargedparticles from the sample, the moving rate of the scanning back processcan be significantly higher, which induces shorter moving period.Therefore, whether the charged particle beam is projected on the sampleduring the scanning back process is minor, the effect of the projectedcharged particle beam is small owing to the projection period on a pointis shorter. Accordingly, the invention does not limit whether thecharged particle beam is blanked out or turned off during the scanningback process, although it usually is not blanked out or not turned offfor the conventional “tilt scan”.

Furthermore, the microstructures inside an image area sometimes haveperiodical configuration along a direction parallel to or vertical tothe orientation of the image area. In such situation, the periodicalproperty of the microstructures can be simply used to analyze thedifferences between them by using a traditional image comparisonalgorithm. For example, shift the original image one or multiple periodsalong the horizontal or vertical pixel direction (either edge of image)and then comparing with the original image. However, when the image areais scanned along a direction neither parallel to nor vertical to theorientation of the image area, the periodic orientation of patterns inthe acquired tilted image is neither parallel nor vertical to theorientation of the image edges. Then, it will be difficult to compareperiodic patterns using traditional image comparison process as there isno simple periodicity in the orientation parallel or vertical to imageedges. Therefore, in order to be compatible with traditional imagecomparison process periodic pattern for inside image, the invention canfurther have a step and a device for rotating the tiled image into anon-tilted image. For example, if the angle between the tilt scandirection and the orientation of the image area is Θ (such as 45°),then, the tilted image can rotated an angle −Θ (such as −45°) to form anon-tiled image. Herein, the patterns at the non-tilted angle will havea periodicity which proportional to the periodicity of themicrostructures. Then, the conventional image comparison processes usedby the conventional non-tilt scan can be applied to analyze the rotatednon-tilted image. Finally, it should be emphasized that the inventiondoes not and need not to limit the details of how to rotate the tiltedimage into a non-tiled image, and also does not and need not limit thedetails and variation of how the image process adopt to the none-tiltedimage comparison.

In the embodiments presented below, the discussion will be focused onhow to control the scanning route for achieving an effective scan areajust larger enough to delivery an image over an image area, and how tocontrol the timing of beam projection module so that charged particlebeam can only be projected to the sample during period the chargedparticle beam is scanning inside the image area. The general conceptsare focused on the implementation of tilted scan. Other relatedimplementations, for instance, how the sampling area is divided based onimage size, how the image size and shape changes with the tilted angle,how the practical tilted scan angle is decided, and so on, are omitted.Furthermore, in the following embodiments, the image areas areillustrated as thin continuous lines and the scan area is illustrated asthin dashed lines. Moreover, the scan lines of the charged particle beamis illustrated as bold continuous lines when the charged particle beamis projected on the image area and illustrated as bold dashed lines whenthe charged particle beam is not projected on the image area.

FIGS. 3A to 3H illustrate a series of steps of a method for examining asample with scanning charged particle beam imaging apparatus accordingto an embodiment of the present invention. In the present embodiment,the sample is examined by the following steps that a scan area and acorresponding image area are raster scanned by using numerous scanlines.

Referring to FIG. 3A, assuming the scanning charged particle beamimaging apparatus can reach a tilted scan area B over which an image ofnone-tilted area A inside B can be formed. The scan area B is ofrectangular shape and defined by four sides E1, E2, E3, E4 and while theimage area A is also of rectangular shape and defined by four side E1′,E2′, E3′, E4′. Usually the image area A can be fully contained insidethe scan area B, but at the situation of maximum size, the scan area Bmay intersect with image area A at least two vertices V1′ and V3′ ofopposite sides E1 and E3 as indicated in the figure. Also in FIG.3A˜FIG. 3H, the scan direction is assumed to be parallel to sides E1 andE3, as indicate by scan direction D1, each line scans starts from sideE4 and ends at side E2 for collecting image signals, and line to linehas a fixed offset in the direction perpendicular to the line scandirection D1.

Referring to FIG. 3B, at step 1, when scanning along the first directionD1 from the vertex V1 to the vertex V1′ of the image area A, the chargedparticle beam is blanked out or turned off. Thereafter, at step 2, whenscanning through the vertex V1′, the charged particle beam is projectedon the image area A.

Referring to FIG. 3C, at step 3, when scanning along the first directionD1 from the vertex V1′ to the vertex V2, the charged particle beam isblanked out or turned off. This finishes the first line scan.

Thereafter, referring to FIG. 3D, at step 4, scan back in thesubstantially opposite direction to D1 from ending point V2 of firstscan line on side E2 to the starting point P1 of second scan line onside E4, wherein the first scan line is in parallel to second scan lineand has an fixed offset of distance d which corresponds to the pixelsize of image. Herein, the charged particle beam could be blanked out orturned off during the moving back from the end of the previous scan lineto the beginning of the second scan line, and the real trajectory of thecharged particle beam during blanking is not limited thus not shown inthe figures.

Referring to FIG. 3E, at step 5, when scanning along the first directionD1 from the first point P1 to the edge E1′ of the image area A, thecharged particle beam is blanked out or turned off.

Next, referring to FIG. 3F, at step 6, when scanning along the firstdirection D1 within the image area A from the edge E1′ to another edgeE2′ of the image area A, the charged particle beam is projected on theimage area A.

Next, referring to FIG. 3G, at step 7, when scanning along the firstdirection D1 from the edge E2′ to the second point P2 on the edge E2,the charged particle beam is blanked out or turned off.

Thereafter, referring to FIG. 3H, at step 8, scans back from the endingpoint of second scan line on edge E2 to the starting point P3 of thethird scan line on the edge E4, wherein the starting point P3 of thethird scan line is away from the starting point P2 of the second scanline in a distance d, which usually is also equivalent to the imagepixel size. Herein, again, the charged particle beam could be blankedout or turned off during the moving back, and the real trajectory of thecharged particle beam is not restricted during the scanning back.

Accordingly, the scan area B may be entirely scanned by a raster scanthat all scan lines are scanned in sequence with uniform distancebetween neighboring scan lines as described in the previous steps.Herein, to easily implement and use for image process, as usual, equaldistances and equal pixel size is used to get a none distorted image.However, for the main characteristics of this invention, all distancesamong neighboring points can be equal or non-equal, and although allpoints can be listed along the edge E4 in sequence or in any order.

Noted that the charged particle beam may not only scan from the edge E4to the edge E4′ but also may from the edge E1 to another vertex V3′ oranother edge E3′ of the image area A when repeating the step 5. Inaddition, the charged particle beam may not only scan from the edge E4′to the edge E1′ but also may scan from the edge E4′, the vertex V3′ orthe edge E3′ to the edge E1′, another vertex V2′ or another edge E2′ ofthe image area A when repeating the step 6. Furthermore, the chargedparticle beam may not only scan from the edge E1′ but also from thevertex V2′ or the edge E2′ to the edge E2 when repeating the step 7.Therefore, the charged particle beam may scan the whole of the scan areaB by repeating the steps 5 to 7 until the whole scan area B is scannedalong the second direction D2 and essentially only the actual image areais projected by the charged particle beam. Thereby, the embodiment caneffectively reduce the possibility of disadvantage on any portion of anyneighboring image areas close to the image area A and overlapped withthe image area B used for scanning the image area A. In summary, in oneexample the charged particle beam scans the scan area B along the firstdirection D1 at a tilted angle neither parallel nor perpendicular toedges of the image area A, and then the possibility of incurrence of analiasing effect is effectively reduced.

FIGS. 4A to 4F illustrate a series of steps of a method for examining asample with scanning charged particle beam imaging apparatus accordingto another embodiment of the present invention. The present embodimentis similar with the previous embodiment illustrated in FIGS. 3A to 3Hthat a scan area and a corresponding image area are raster scanned bynumerous scan lines. However, in this embodiment described here, eachscan line is offset with a give interval in a direction notperpendicular to the line scan direction. Moreover, the line to lineoffset is set in the direction parallel the die orientation.

Referring to FIG. 4A, in more details, the image area A is overlappedwith a scan area B, both areas have a different shape, but with twocommon edges facing opposite, for instance, E2′ and E4′ of the imagearea A overlapped with the edge E2 and the edge E4 of the scan area Brespectively. Hereinafter, the process for scanning the scan area B bythe charged particle beam is described as follows.

Referring to FIG. 4B, at step 1, when scanning through the vertex V1,the charged particle beam is projected on the image area A. Next, atstep 2, when scanning along the first direction D1 from the vertex V1 tothe vertex V2, the charged particle beam is blanked out or turned off.

Thereafter, referring to FIG. 4C, at step 3, scan back to a first pointP1 on the edge E4′, wherein the first point P1 is a first distance daway from the vertex V1. Again, the charged particle beam could beblanked out or turned off during the scanning back, and the realtrajectory of the charged particle beam is not limited during thescanning back.

Referring to FIG. 4D, at step 4, when scanning along the first directionD1 within the image area A from the first point P1 to the edge E1′, thecharged particle beam is projected on the image area A.

Next, referring to FIG. 4E, at step 5, when scanning along the firstdirection D1 from the edge E1′ to the second point P2 on the edge E2′,the charged particle beam is blanked out or turned off.

Thereafter, referring to FIG. 4F, at step 6, scan back to a third pointP3 on the edge E4′, wherein the third point P3 also is a distance awayfrom the first point P1. Again, the charged particle beam could beblanked out or turned off during the scanning back, and the realtrajectory of the charged particle beam is not limited during thescanning back. Herein, although all distance are equal in thisembodiment, the invention does not particularly restrict this.Similarly, although all points listed in sequence along the edge E4′ andE2′ in this embodiment, the invention does not particularly restrictthis.

Accordingly, after that, by repeating the above steps, the whole scanarea B is scanned by a raster scan that all scan lines are scanned insequence. Moreover, by comparing this embodiment with the perviousembodiment, it is clear that the invention does not limit how the scanarea 104 encloses the image area 103.

Furthermore, because how the scan area 104 encloses the image area 103is not limited, the relative geometric relation between the scan area104 and the image area 103 is variable. Hence, the steps 4 and 5illustrated in FIGS. 4D and 4E may be replaced by three scanning schemeswith the steps respectively illustrated in FIGS. 4G to 4I. Note thatwhen the charged particle beam scans along the first direction D1 fromthe edge E4′ to the edge E2′, the scanning steps are not limited toprocessing the steps 4 and 5 sequentially.

In more detail, referring to FIG. 4G, after scanning through the vertexV4′, the scan line of the charged particle beam may be overlapped withthe image area A. Hence, instead of executing the steps 4 and 5, thecharged particle beam is continuously projected on the image area A whenscanning along the first direction D1 from the edge E4′ to the edge E2′.

In addition, referring to FIG. 4H, after scanning through the vertexV4′, the starting point of the scan line of the charged particle beam(illustrated as bold dashed lines) may be located outside the image areaA, and the ending point thereof (illustrated as bold continuous s) maybe confined within the image area A. Therefore, instead of executing thesteps 4 and 5, the charged particle beam is blanked out or turned offfirstly when scans the starting portion of the scan line and thenprojected on the image area A when scans the other portion of the scanline.

Furthermore, when the charged particle beam scans the scan area B alonga first direction D1 illustrated in FIG. 4I different from the firstdirection D1 illustrated in FIGS. 4A to 4H, the scanning steps afterscanning through the vertex V4′ may be different from that illustratedin FIG. 4H. In more detail, referring to FIG. 4I, after scanning throughthe vertex V4′, both the starting portion and the ending portion of thescan line of the charged particle beam (illustrated as bold dashedlines) may be located outside the image area A, and the middle portionthereof (illustrated as bold continuous lines) may be confined withinthe image area A. Therefore, instead of executing the steps 4 and 5, thecharged particle beam is blanked out or turned off first when scanningon the starting portion of the scan line. Then, projected on the imagearea A when scanning on the middle portion of the scan line, and finallyblanked out or turned off again when scanning on the ending portion ofthe scan line. Therefore, the charged particle beam may scan the wholescan area B by repeating the steps illustrated above until the wholescan area B is scanned along the second direction D2.

Accordingly, the charged particle beam may scan the whole scan area B byrepeating the above steps until the all scan lines on scan area B arescanned along the second direction D2 in sequence, and also by beingprojected essentially only on the actual image area. Thereby, theembodiment can effectively reduce the possibility of disadvantageousnesson any neighboring image areas being close to the image area A andoverlapped with the image area B. In addition, the charged particle beamscans the scan area B along the first direction D1 at a tilted angleneither parallel nor perpendicular to edges of the image area A, andthen the possibility of incurrence of an aliasing effect is effectivelyreduced.

FIGS. 5A to 5G illustrate a series of steps of a method for examining asample with scanning charged particle beam imaging apparatus accordingto another embodiment of the present invention. The present embodimentis similar with the previous embodiments illustrated in FIGS. 3A to 3Hand FIGS. 4A-4I that a scan area and a corresponding image area areraster scanned by numerous scan lines. However, in the embodimentdescribed here, the relative geometric relation between the scan areaand the image area is different than that of the previous describedembodiments. In the present embodiment, the sample 100 illustrated inFIG. 1 may be examined by the following steps.

Referring to FIG. 5A, a scan area B located on the surface of the samplemust be defined first. Herein, the image area A is entirely covered bythe scan area B. Herein, a shape of the scan area B is essentiallysimilar with a shape of the image area A and an area of the scan area Bis also essentially similar with an area of the image area A (the sizeof the scan area is at most lightly larger than the size of the imagearea). In other words, the overlapped size between the scan area B andthese neighboring image areas close to the image area A is significantlysmaller than the overlapped size between scan area B and the image areaA. Thereafter, the charged particle beam may scan through whole the scanarea B along a direction D3 that is at a tilted angle neither parallelnor perpendicular to an orientation of the image area A. Pleaseparticularly note that the invention also allows the scan area B isentirely equal to the image A, both the shape and the size, if thepractical scanning charged particle beam imaging apparatus caneffectively adjust the scan lines used by the scan area B to preciselyfit the image area A.

Then, referring to FIG. 5B, the charged particle beam is scanned fromthe edge E4 along the direction D3 through a portion of the image area Ato the edge E1, wherein the portion of the image area A may be thevertex V1′ or a portion close to the vertex Vr. Herein, the direction D3is at a tilted angle neither parallel nor perpendicular to at least oneof the edge E1, the edge E4, the edge E1′ and the edge E4′. For example,the direction D3 could be parallel to a line linking two vertexes V2′and V4′.

Next, referring to FIG. 5C, the charged particle beam is scanned back toa first point P1 on the edge E4, wherein the first point P1 is adistance d away from where the previous scanning step is started. How tomove to the P1 is not limited in this embodiment. For example, thecharged particle beam can be moved back to the edge E4 along a fourthdirection D4 opposite to the direction D3 as illustrated in FIG. 5B, andmoved to a distance d along the second direction D2 to the first pointP1 on the edge E4. For example, the charged particle beam can be blankedout or turned off during the scanning back.

Thereafter, referring to FIG. 5D, the charged particle beam is scannedfrom the first point P1 along the direction D3 through another portionof the image area A to the edge E1.

Then, referring to FIG. 5E, the charged particle beam is scanned back toa second point P2 on the edge E4, wherein the second point P1 also is adistance d away from the first point P1. For example, the chargedparticle beam may be scanned back to the first point P1 along the fourthdirection D4, and moved to the distance d along the second direction D2to a second point P2 on the edge E4. For example, the charged particlebeam can be blanked out or turned off during the scanning back. Afterthat, the steps illustrated above may be continuously repeated until thewhole scan area B is scanned. Clearly, as discussed in the aboveembodiments, the embodiment does not restrict the details of thesepoints, such as the distance between neighboring points and thearrangement of these points. Indeed, the only limitation is that allpoints are located on the edge E4. Besides, as discussed in the aboveembodiment, the embodiment also does not restrict how to move theprojection of charged particle beam from an ending point of a previousscan route to the starting point of a next scan route.

Next, referring to FIG. 5F, after scanning through a vertex V4 and/or avertex V2, the charged particle beam may further optionally scan alongthe direction D3 from the edge E4/E3 to the edge E1/E2. Herein, thevertex V4 is an interaction of the edge E4 and the edge E3, and thevertex V2 is an interaction of the edge E1 and the edge E2. Clearly, thepractical details of the step illustrated in FIG. 5F is at leastdependent on the length of the edge E4, the length of the edge E1, andtilted angle is used.

Thereafter, referring to FIG. 5G, the charged particle beam may bescanned back to the edge E3. For example, it may be scanned along thefourth direction D4 to the edge E3, and then moved a distance d along afifth direction D5. Herein, the fifth direction D5 is parallel to theedge E3 and the charged particle beam is optional blanked out or turnedoff during the scanning back. Accordingly, the charged particle beam mayscan the whole scan area B by repeating the steps illustrated aboveuntil the whole scan area B is scanned along the second direction D2 andthe fifth direction D5.

Noted that the charged particle beam may further be blanked out orturned off when be scanned outside the image area A but inside any ofthe neighboring image area being close to the image area A. In otherun-illustrated embodiments, the charged particle beam may further scanthe scan area B along the direction D3 from the edge E4 to the edge E2,or from the edge E3 to the edge E1, for example in the case of samplewith a shape a high aspect ratio rectangle. Accordingly, the methoddisclosed in the present embodiment may not only reduce the possibilityof damaging other image areas being close to the image area A and analiasing effect when scanning on the edges of microstructures inside theimage area A, but also increase the throughput of the tilted scanprocess for examining the sample (Please particularly compare the sizeof overlapped portion of the neighboring image area with the size of thewhole image area A).

In summary, the methods disclosed in the present invention may not onlyreduce the possibility of causing disadvantageousness on neighboringimage areas close to the image area desired to be scanned, but alsoreduce the possibility of incurrence of an aliasing effect when scanningon the edges of microstructures inside the image area. In addition, themethod disclosed in the present invention may increase the throughput ofthe scanning charged particle beam imaging apparatus.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method for examining a sample with scanning charged particle beamimaging apparatus, comprising: specifying an image area and a scan area,wherein said image area is a portion of a surface of said sample andsaid scan area is an area to be scanned, wherein said image area isentirely covered with said scan area, wherein an orientation of saidscan area is at a tilted angle neither parallel nor perpendicular to anorientation of said image area; and scanning said scan area, whereinsaid charged particle beam is moved in parallel to said orientation ofsaid scan area, wherein a portion of said scan area outside said imagearea is essentially not exposed to said charge particle beam, whereinwhen said scan area is a quadrangle with a edge along a first directionand a edge along a second direction and said image area is a quadranglelocated inside said scan area and with its two opposite vertexes locatedon said edge and an edge opposite to said edge, said scan area isscanned according to the following process steps: (A1) blanking saidcharged particle beam when scanning along said first direction from aproximal end of said edge to a predetermined point on said edge, saidproximal end being on an intersection of said first and edges, saidpredetermined point being on a proximal vertex of said image area; (A2)projecting said charged particle beam on said image area when scanningthrough said proximal vertex of said image area; (A3) blanking saidcharged particle beam when scanning along said first direction from saidcertain point of said edge to a distal end of said edge, said distal endbeing on an intersection of said edge and an edge opposite to said edge;(A4) scanning back to a first point of said edge, said first point beinga first distance away from said proximal end; (A5) blanking said chargedparticle beam when scanning along said first direction until scanning toan edge or a vertex of said image area; (A6) projecting said chargedparticle beam on said image area when scanning along said firstdirection within said image area until scanning to an another edge oranother vertex of said image area; (A7) blanking said charged particlebeam when scanning along said first direction from said another edge orsaid another vertex of said image area to said edge of said scan areaopposite to said edge; (A8) scanning back to a second point of saidedge, said second point being a second distances away from said firstpoint: and (A9) repeating said steps (A5) to (A8) until said chargedparticle beam completely scan said scan area along said seconddirection.
 2. The method as claimed in claim 1, wherein said chargedparticle beam is provided by a machine and said scan area is not largerthan the maximum area that can be viewed by using said charged particlebeam at the same time.
 3. The method as claimed in claim 1, when saidimage area is a portion of a sampling region on said surface of saidsample, further comprising a step of dividing said sampling region intoa plurality of sub-regions, wherein each said sub-region could be viewedas a said image area being smaller than said scan area.
 4. The method asclaimed in claim 1, wherein a portion of said scan area proximate tosaid image area is exposed to said charged particle beam and a portionof said scan area far away from said image area is not exposed to saidcharged particle beam.
 5. The method as claimed in claim 1, furthercomprising a step of checking whether a portion of said scan area isoverlapped within said image area before scanning said portion of saidscan area, and also further comprising a step of blanking said chargedparticle beam before scanning said portion of said scan area when saidportion does not lie within said image area.
 6. The method as claimed inclaim 1, further comprising a step of reconstructing a non-tilted imagefrom said scan area as if said charged particle beam scans in parallelto the orientation of pattern of said sample.
 7. The method as claimedin claim 1, further comprising a step of blanking said charged particlebeam in at least one of the following steps: (A4) and (A8).
 8. A methodfor examining a sample with scanning charged particle beam imagingapparatus, comprising: specifying an image area and a scan area, whereinsaid image area is a portion of a surface of said sample and said scanarea is an area to be scanned, wherein a portion of said image area isentirely covered with said scan area, wherein an orientation of saidscan area is at a tilted angle neither parallel nor perpendicular to anorientation of said image area and scanning said scan area, wherein saidcharged particle beam is moved in parallel to said orientation of saidscan area, wherein another portion of said scan area outside said imagearea is essentially not exposed to said charge particle beam, whereinwhen said scan area is a quadrangle with a edge along a first directionand a edge along a second direction and said image area is a quadranglelocated inside said scan area and with its two opposite edgesoverlapping said edge and an edge opposite to said edge of said scanarea, said scan area is scanned according to the process steps: (B1)projecting said charged particle beam on said proximal end of said edge,said proximal end being on a proximal vertex of said image area, saidproximal end being on an interaction of said edge and said edge; (B2)blanking said charged particle beam when scanning along said firstdirection from said proximal end of said edge to a distal end of saidedge, said distal end of said edge being on an interaction of said edgeand an edge opposite to said edge; (B3) scanning back to a third pointof said edge, said third point being a third distance away from saidproximal end; (B4) scanning along said first direction from said edge tosaid edge opposite to said edge, wherein said charged particle beam isprojected when scanning within said image area and is blanked whenscanning outside said image area; (B5) scanning back to a fourth pointof said edge, said fourth point being a fourth distance away from saidthird point; and (B6) repeating said steps (B4) to (B5) until saidcharged particle beam completely scans said scan area along said seconddirection.
 9. The method as claimed in claim 8, wherein said step (B4)is selected from a group consisting of the following steps: whenscanning from a portion of said edge overlapped within said image areato a portion of said edge opposite to said edge located outside saidimage area, first projecting said charged particle beam when scanningwithin said image area and then blanking said charged particle beam whenscanning outside said image area; when scanning from a portion of saidedge overlapped within said image area to a portion of said edgeopposite to said edge overlapped within said image area, continuouslyprojecting said charged particle beam; when scanning from a portion ofsaid edge located outside said image area to a portion of said edgeopposite to said edge overlapped within said image area, first blankingsaid charged particle beam when scanning outside said image area andthen projecting said charged particle beam when scanning within saidimage area; and when scanning from a portion of said edge locatedoutside said image area to a portion of said edge opposite to said edgelocated outside said image area, first blanking said charged particlebeam when scanning outside said image area, then projecting said chargedparticle beam when scanning within said image area and finally blankingsaid charged particle beam when scanning outside said image area again.10. The method as claimed in claim 8, further comprising a step ofblanking said charged particle beam is at least one of the followingsteps: (B3) and (B5).
 11. A method for examining a sample with scanningcharged particle beam imaging apparatus, comprising: specifying an imagearea and a scan area, wherein said image area is a portion of a surfaceof said sample and said scan area is an area to be scanned, wherein saidimage area is entirely covered with said scan area, wherein a shape ofsaid scan area is similar with a shape of said image area, and wherein asize of said scan area is at most slightly larger than a size of saidimage area; and scanning said scan area, wherein said charged particlebeam is moved at a tilted angle neither parallel nor perpendicular to anorientation of said image area, wherein when said scan area is aquadrangle angle with a edge along a first direction and a edge along asecond direction, and when said image area is a quadrangle angle with aedge along a third direction and a edge with a fourth direction, saidscan area is scanned by said charged particle beam according to thefollowing process steps: (C1) scanning from said edge along said fifthdirection through a first portion of said image area to said edge,wherein said first portion of said image area is proximate to or equalto a proximal vertex of said image area, said proximal vertex is aninteraction of said edge and said edge; (C2) scanning back to a firstpoint of said edge, said first point being a first distance away from apoint where said scanning in step (C1) is started; (C3) scanning fromsaid edge along said fifth direction through a second portion of saidimage area to said edge; (C4) scanning back to a second point of saidedge, said second point being a second distance away from said firstend; and (C5) repeating said steps (C3) and (C4) until said chargedparticle beam completely scans said image area along said seconddirection.
 12. The method as claimed in claim 11, wherein said chargedparticle beam is provided by a scanning charged particle beam imagingapparatus and said scan area is not larger than the maximum area thatcan be viewed by using said charged particle beam at the same time. 13.The method as claimed in claim 11, when said image area is a portion ofa sampling region over said surface of said sample, further comprising astep of dividing said sampling region into a plurality of sub-regions,wherein each said sub-region could be viewed as a said image area. 14.The method as claimed in claim 11, further comprising a step ofreconstructing a non-tilted image from said scan area as if said chargedparticle beam scans in parallel to the orientation of pattern of saidsample.
 15. The method as claimed in claim 11, further comprising a stepof blanking said charged particle beam in said steps (C2) and (C4). 16.The method as claimed in claim 11, further comprising blanking saidcharged particle beam when said charged particle beam is moved outsidesaid image area during the required transferring periods between saidstep (C1) and step (C2), step (C2) and step (C3), and step (C3) and step(C4).
 17. The method as claimed in claim 11, wherein said fifthdirection is neither parallel nor perpendicular to said third directionand said fourth direction.
 18. The method as claimed in claim 11,wherein said fifth direction is parallel to a line linking two vertexesof said image area, wherein said two vertexes does not include saidproximal vertex and a vertex opposite to said proximal vertex.
 19. Themethod as claimed in claim 11, further comprising a step of scanningalong said fifth direction to a fifth edge of said scan area when thesum of all movements along said edge is longer than a length of saidedge, wherein said fifth edge is connected to said edge.
 20. The methodas claimed in claim 11, further comprising a step of moving back to anadditional point of a sixth edge being connected to said edge when thesum of all movements along said edge is longer than a length of saidedge, wherein said additional point is an additional distance away fromlast said point on said edge.